In video apparatus, such as television receivers or monitors, video cassette recorders, video disc recorder/player units and the like, the need frequently arises for separating a composite video signal into individual luminance and chrominance components. A simple way of providing such separation is to low-pass filter the composite signal to obtain a luminance component and to high-pass or band-pass filter the composite signal to obtain the chrominance component. This technique, however, does not efficiently recover the components and results in a loss of sharpness in displayed images. Since composite components are spectrally interleaved, comb filtering provides a more efficient separation and so a clearer displayed picture.
The advantages of comb filter separation of the luminance (Y) and chrominance (C) components of a composite video signal are well known. In the most elementary "one-line" or "1-H" form of comb filter, picture elements ("pixels" hereafter) temporally spaced a line apart are added to produce a separated luminance component and subtracted to produce a separated chrominance component. Such filters provide superior image detail as compared, for example, with the low-pass/high-pass filter method of Y/C separation but may tend to exhibit visual artifacts (e.g., hanging dots) for certain image features.
Some two-line (2-H) comb filters provide enhanced performance over the 1-H comb described above by "adapting" the filter to the image detail and so achieve a desirable reduction in visual artifacts. In principle, this is done by combing the composite signal twice in the vertical direction to produce two combed chrominance signals and then selecting or "blending" them with a "soft-switch" based on an analysis of image characteristics to thereby select the signal (or "blend" of signals) having the least visual artifacts. The chrominance signal so produced is then subtracted from the composite signal to provide a separated luminance output signal. This form of chrominance/luminance signal separation is commonly known as "2-H" or two-line combing and provides reduced visual artifacts as compared with 1-H combing and improved image detail as compared with the simple low-pass/high-pass filtering method of Y/C separation. A 2-H comb filter Y/C separation circuit is described, for example, by McNeely and Willis in U.S. Pat. No. 4,786,963 entitled ADAPTIVE Y/C SEPARATION APPARATUS FOR TV SIGNALS which issued 22 Nov. 1988.